1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and a printer.
2. Description of the Background Art
Image forming apparatuses such as a copying machine and a printer generally employs electrophotographic system, in which a surface of an electrostatic latent image carrier is uniformly charged, and an electrostatic latent image is formed by effecting image exposure corresponding to an original image or image data on a charged region of the image carrier surface. The electrostatic latent image thus formed is developed into a visible image, which is immediately transferred onto a transfer member and is fixed. In a color image forming apparatus, the visible image is transferred onto an intermediate image carrier (intermediate transfer member), and then is transferred from the intermediate image carrier onto a transfer member and is fixed.
A charging device is used for charging the surface of the electrostatic latent image carrier or charging the intermediate image carrier for the purpose of transferring the visible image onto the intermediate image carrier.
Various types of charging devices have been proposed and available. Some types of the charging devices employ charging members, each of which has a contact portion in contact with the surface of the image carrier and charges a predetermined surface by performing discharging from a discharge contributing surface other than the contact portion, or employ charging members, each of which is spaced from the surface of the image carrier surface by a predetermined short distance, and charges the predetermined surface by performing discharging from the discharge contributing surface. In these types of charging devices, however, the charging surface may be smeared by toner for development, paper powder, adhesive or the like adhering onto the discharge contributing surface due to some reasons. This smear may impede smooth and uniform discharging, resulting in noises on images.
For example, an image forming apparatus of a cleanerless type is known. In this apparatus, a dedicated cleaner is not used, but developer which remains on the electrostatic latent image carrier without being transferred onto the transfer member is removed and collected by a developing/cleaning device simultaneously with development. In the image forming apparatus of this type, the discharge contributing surface of the charging member is often smeared by untransferred residual developer, paper powder, adhesive or the like which remain on the electrostatic latent image carrier even after the transfer of the visible image from the electrostatic latent image carrier onto the transfer member or the intermediate image carrier. In the image forming apparatus provided with the dedicated cleaner for removing untransferred residual developer from the image carrier, the discharge contributing surface of the charging member is often smeared by the developer, paper powder and others which passed through the cleaner.
For overcoming the above problems, it has been proposed, for example, to add a cleaning device for the charging member to the charging device. Also, it has been proposed to add a device for reducing a pressure of the charging member against the image carrier or add a device for making a space between them in order to allow easy passage of the untransferred residual developer between the charging member and the image carrier. In these manners, smear of the charging members can be suppressed.
However, these proposals, i.e., addition of the device such as a cleaning device, a pressure reducing device or a space making device to the charging device, unpreferably increase sizes of the image forming apparatus, complicate the structure thereof, and therefore increase a manufacturing cost.
Many image forming apparatuses of the cleanerless type employ charging members of a brush type as devices for charging the image carriers prior to image exposure. The purpose thereof is to disperse the untransferred residual toner by the charging member of the brush type so that the residual toner may not cause disadvantages such as exposure eclipse in the next image exposure for forming the electrostatic latent image.
The charging member of the brush type is used for shaving off the surface of the image carrier.
The shaving of the surface of the image carrier is required for shaving off discharge products, i.e., materials produced by discharging if the image carrier is an organic photosensitive member. More specifically, discharge products such as ozone produced by discharging from the charging device adhere onto the photosensitive member, and the adherent materials absorbs moisture so that the surface of the photosensitive member becomes electrically conductive, and this conductive state causes failure in image exposure. Regardless of whether the image carrier is an organic photosensitive member or not, toner adhered onto the surface of the image carrier forms a film due to the cleanerless structure. This toner film impedes the image exposure so that appropriate shaving of the surface of the image carrier is required for preventing this in addition to the foregoing purpose.
However, in the conventional image forming apparatus of the cleanerless type in which the image carrier surface is shaved off, the shaved surface of the image carrier is liable to have a high roughness. In particular, the charging member of the brush type tends to increase the roughness of the surface shaved thereby. An excessively rough surface roughness of the image carrier causes irregular reflection of light beams during the image exposure, and therefore impedes desired image exposure. For example, in an image forming apparatus performing reversal development, the exposure cannot sufficiently lower a potential on a portion to which toner is to be adhered so that this portion insufficiently carry the toner, resulting in an insufficient density at a final image.
If the shaving is performed to produce an excessively large roughness of the image carrier surface, irregularities in surface roughness occur at various portions on the image carrier surface, which causes irregularities in density at the final image. Locally deep and shallow portions are liable to be formed by the shaving, resulting in stripe noises on the final image.
The foregoing developing/cleaning device in the image forming apparatus of the cleanerless type removes and collects the untransferred residual toner, which still remains on the image carrier surface after the transfer of the visible toner image onto the transfer member, onto the developer carrier by utilizing a potential difference between a developing bias voltage applied to the developer carrier in the developing/cleaning device and the surface potential of the image carrier. For example, in the reversal development, the electrostatic latent image region on the image carrier surface, which carries the potential lowered by the image exposure, is electrostatically supplied with developer toner from the developer carrier for development, as is done also in an ordinary reversal development, and simultaneously the residual toner, which remains on the image carrier after the last transfer and particularly stays on a non-image portion not subjected to the exposure, is electrostatically absorbed and collected onto the developer carrier owing to the potential difference between the surface potential on the image carrier charged by the charging device and the developing bias.
However, various kinds of transfer sheets or members are now available for use in copying machines and printers. Some of them may contain special materials at their surfaces which affect the image carrier or its peripheral equipments. Also, some kinds of transfer members may carry adhesive or a large amount of paper powder thereon. If these kinds of transfer members are used many times, foreign matters such as special materials, adhesive or paper powder adhere onto the image carrier so that black or blank spots appear in the final image. This is for the following reason. Since the charging device cannot charge the image carrier portions carrying such foreign matters so that the developing/cleaning device develops these portions in spite of the fact that these portions are non-image portions. Alternatively, regions which are to be developed but carry the foreign matters cannot carry the toner transferred thereto so that white blanks are formed. If the foreign matters are large in size, white blanks are formed also around those regions.
The image forming apparatuses employ charge applying devices or charging devices for charging the image carrier surface, for example, as described above. Among these charge applying devices, charge applying devices of a contact type are advantageous and, particularly, devices provided with charging members of a sheet-like or brush-like form are more advantageous than devices employing roller-like form in view of small and simple structures. However, these types of devices may suffer from variation and irregularity in charge applying performance due to deposition of foreign matters such as paper powder or toner at its portion which is in contact with a charge target member, i.e., member to be charged, and therefore requires appropriate measures against it. As already discussed and as disclosed, for example, in Japanese Laid-Open Patent Publication No. 4-184359 (184359/1992), some conventional charge applying devices are provided with mechanisms for removing foreign matters by releasing the charging member from the charge target member.
However, provision of the mechanism for releasing the pressure complicates the structure of the apparatus, which may cancel the advantage achieved by employing the charging member in a sheet or brush form. However, if the pressure is not released, it is impossible to remove sufficiently the foreign matters adhering to or deposited on the portions of the charging member in contact with the charge target member so that variation or irregularity in charging performance may occur. This may result in stripe noises in images produced by the image forming apparatus.